MULTI-IONIC LITHIUM SALTS FOR USE IN SOLID POLYMER ELECTROLYTES FOR LITHIUM BATTERIES
AuthorChinnam, Parameswara Rao
AdvisorWunder, Stephanie L.
Committee memberWayland, Bradford B.
Zdilla, Michael J., 1978-
Multi-ionic Lithium Salts
Polymer Lithium Batteries
Solid Polymer Electrolytes
Permanent link to this recordhttp://hdl.handle.net/20.500.12613/2692
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AbstractCommercial lithium ion batteries use liquid electrolytes because of their high ionic conductivity (>10-3 S/cm) over a broad range of temperatures, high dielectric constant, and good electrochemical stability with the electrodes (mainly the cathode cathode). The disadvantages of their use in lithium ion batteries are that they react violently with lithium metal, have special packing needs, and have low lithium ion transference numbers (tLi+ = 0.2-0.3). These limitations prevent them from being used in high energy and power applications such as in hybrid electric vehicles (HEVs), plug in electric vehicles (EVs) and energy storage on the grid. Solid polymer electrolytes (SPEs) will be good choice for replacing liquid electrolytes in lithium/lithium ion batteries because of their increased safety and ease of processability. However, SPEs suffer from RT low ionic conductivity and transference numbers. There have been many approaches to increase the ionic conductivity in solid polymer electrolytes. These have focused on decreasing the crystallinity in the most studied polymer electrolyte, polyethylene oxide (PEO), on finding methods to promote directed ion transport, and on the development of single ion conductors, where the anions are immobile and only the Li+ ions migrate (i.e. tLi+ = 1). But these attempts have not yet achieved the goal of replacing liquid electrolytes with solid polymer electrolytes in lithium ion batteries. In order to increase ionic conductivity and lithium ion transference numbers in solid polymer electrolytes, I have focused on the development of multi-ionic lithium salts. These salts have very large anions, and thus are expected to have low tanion- and high tLi+ transference numbers. In order to make the anions dissociative, structures similar to those formed for mono-ionic salts, e.g. LiBF4 and lithium imides have been synthesized. Some of the multi-ionic salts have Janus-like structures and therefore can self-assemble in polar media. Further, it is possible that these salts may not form non-conductive ion pairs and less conductive ion triplets. First, we have prepared nanocomposite electrolytes from mixtures of two polyoctahedral silsesquioxanes (POSS) nanomaterials, each with a SiO1.5 core and eight side groups. POSS-PEG8 has eight polyethylene glycol side chains that have low glass transition (Tg) and melt (Tm) temperatures and POSS-phenyl7(BF3Li)3 is a Janus-like POSS with hydrophobic phenyl groups and -Si-O-BF3Li ionic groups clustered on one side of the SiO1.5 cube. The electron-withdrawing POSS cage and BF3 groups enable easy dissociation of the Li+. In the presence of polar POSS-PEG8, the hydrophobic phenyl rings of POSS-phenyl7(BF3Li)3 aggregate and crystallize, forming a biphasic morphology, in which the phenyl rings form the structural phase and the POSS-PEG8 forms the conductive phase. The -Si-O-BF3- Li+ groups of POSS-phenyl7(BF3Li)3 are oriented towards the polar POSS-PEG8 phase and dissociate so that the Li+ cations are solvated by the POSS-PEG8. The nonvolatile nanocomposite electrolytes are viscous liquids that do not flow under their own weight. POSS-PEG8/POSS-phenyl7(BF3Li)3 at O/Li = 16/1 has a conductivity, σ = 2.5 x 10-4 S/cm at 30°C, 17 x greater than POSS-PEG8/LiBF4, and a low activation energy (Ea ~ 3-4 kJ/mol); σ = 1.6 x 10-3 S/cm at 90°C and 1.5 x 10-5 S/cm at 10°C. The lithium ion transference number was tLi+ = 0.50 ± 0.01, due to reduced mobility of the large, bulky anion and the system exhibited low interfacial resistance that stabilized after 3 days (both at 80°C). Secondly, solid polymer electrolytes have been prepared from the same salt, POSS-phenyl7(BF3Li)3 and polyethylene oxide (PEO). These exhibit high ambient temperature conductivity, 4 x 10-4 S/cm, and transference number, tLi+ = 0.6. A two-phase morphology is proposed in which the hydrophobic phenyl groups cluster and crystallize, and the three -BF3- form an anionic pocket, with the Li+ ions solvated by the PEO phase. The high ionic conductivity results from interfacial migration of Li+ ions loosely bonded to three -BF3- anions and the ether oxygens of PEO. Physical crosslinks formed between PEO/Li+ chains and the POSS clusters account for the solid structure of the amorphous PEO matrix. The solid polymer electrolyte has an electrochemical stability window of 4.6 V and excellent interfacial stability with lithium metal. In order to further enhance the ionic conductivity of solid polymer electrolytes, we have made two improvements. First, we have used so called half cube structures, T4-POSS, that contain 4 phenyl groups on one side of a Si-O- ring, and 4 ionic groups on the other side, and so are true Janus structures. They contain a 4/4 ratio of phenyl/ionic groups, unlike the previous structures that contain 7 phenyl groups/3 ionic groups. At the same O/Li ratio, the ionic conductivity of [PhOSi(OLi)]4 with POSS-PEG8 is higher than POSS-phenyl7Li3 because of more Li+ dissociation in the former case. Second, we have increased the dissociation of the lithium salts by replacing the Si-O-BF3Li groups with Si-(C3H4NLiSO2CF3)4. Both T4-POSS-(C3H4NLiSO2CF3)4 and POSS-(C3H4NLiSO2CF3)8 have been synthesized and characterized, with some preliminary conductivity data obtained.
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HIGH MODULI POLYMER GELS AND NON-AQUEOUS ELECTROLYTES WITH MUTLI-IONIC LITHIUM SALTS HAVING HIGH THERMAL STABILITY AND LITHIUM ION TRANSFERENCE NUMBERSWunder, Stephanie L.; Sun, Yugang; Zdilla, Michael J., 1978-; Sahraei, Elham (Temple University. Libraries, 2020)Rechargeable Li-ion batteries play pivotal role in the growth of the market for portable electronic devices like mobile phones, laptops etc. Several key drawbacks with the Lithium-ion Batteries (LIBs) put a limitation on their use in more advanced applications especially, those that require more power output and rapid charging times such as electric vehicles. One issue is the safety concerns that arise due to the growth of lithium dendrites especially at rapid charging conditions, leading to fire hazards at extreme thermal runaway scenarios. Another issue is that the current state of the art LIBs are fast approaching their maximum theoretical energy density limits. The energy density of the present-day LIBs is insufficient to deliver satisfactory performance especially when used in advanced applications such as electric vehicles which require long driving range per charge. One possible approach suggested to increase the energy density of the battery is by replacing intercalated graphite anode with lithium metal which has greater theoretical capacity. Utilizing lithium metal would open the possibility to use cathodes with conversion chemistry that would store more charge per cycle and there by resulting in the overall increase in the energy density of the battery. One of the key issues impeding the commercialization of Li-metal batteries is safety. Safety issues arises due to the uneven deposition of Li on the surface of the Li metal which would give rise to needle like protrusions (dendrites) that are capable of piercing the separator and reaching all the way to cathode. The result is an internal short circuit of the cell , which ultimately results in fire or explosion. There are many solutions that have been proposed over the years, courtesy of some extensive theoretical and experimental research to tackle the issue of dendrite growth. This thesis describes about the research work along the lines of a couple of possible approaches to prevent the dendrite growth. The first approach to be discussed is the development of solid electrolytes/separators to address this safety issue. The major drawback with most of the solid electrolytes/separators is that they have very low conductivity and lithium ion transference number (tLi+)values. Transference number of an ion by definition is the fraction of current carried by that particular ion, here in the case of LIBs is Li+, out of the total current carried by all the ions in the solution. Ideally a tLi+ of unity is aspired. The goal was to develop solid electrolytes having good room temperature conductivity and high transference numbers and at the same time having good mechanical strength. To that extent high moduli polymer gel electrolytes with high thermal stability were developed by in-situ encapsulation of ionic liquids and solvate ionic liquids into nanofibrillar methyl cellulose networks. The separators/iongels prepared possessed good room temperature conductivity and lithium ion transference numbers. The other approach described is developing non-aqueous liquid electrolytes having high ionic conductivity and high Li+ transference numbers (tLi+). Electrolytes with high tLi+ are efficient in minimizing the concentration polarization and ultimately mitigating the dendrite growth. As a part of this approach a functionalized symmetric, multi-ionic polyhedral oligomeric silsesquioxane (POSS) with dissociative lithium salt (POSS-(LiNSO2CF3)8 ) salt was dissolved in tetraglyme (G4), CH3–O–(CH2CH2O)4–CH3 in a specific O/Li ratio and the solution mixture was used as a electrolyte. Good ionic conductivities (σ10-4 S cm-1) and lithium ion transference numbers of tLi+= 0.65 are achieved in these electrolyte systems.
Improving thermal conduction across cathode/electrolyte interfaces in solid-state lithium-ion batteries by hierarchical hydrogen-bond networkHe, Jinlong; Zhang, Lin; Liu, Ling (2020-09)Effective thermal management is an important issue to ensure safety and performance of lithium-ion batteries. Fast heat removal is highly desired but has been obstructed by the high thermal resistance across cathode/electrolyte interface. In this study, self-assembled monolayers (SAMs) are used as the vibrational mediator to tune interfacial thermal conductance between an electrode, lithium cobalt oxide (LCO), and a solid state electrolyte, polyethylene oxide (PEO). Embedded at the LCO/PEO interface, SAMs are specially designed to form hierarchical hydrogen-bond (H-bond) network with PEO. Molecular dynamics simulations demonstrate that all SAM-decorated interfaces show enhanced thermal conductance and dominated by H-bonds types. The incorporation of poly(acrylic acid) (PAA) SAM drastically enhances interfacial thermal conductance by approximately 211.69%, largely due to the formation of a strong H-bond, -COOH···:O, between PAA and PEO. Even with weaker H-bonds such as -OH···:O, it still outperforms the pristine interface as well as interfaces decorated with non-H-bonded SAMs, e.g. PE. Such improvement is attributed to the unique hierarchical H-bond network at the interface, which removes discontinuities in temperature field, straighten SAM chains, make materials strongly adhere, and couple the vibrational modes of materials. The study is expected to guide surface engineering for more effective thermal management in lithium-ion batteries.
Crystal structures of sodium-, lithium-, and ammonium 4,5-dihydroxybenzene-1,3-disulfonate (tiron) hydratesHerbst-Gervasoni, CJ; Gau, MR; Zdilla, MJ; Valentine, AM; Valentine, Ann|0000-0002-5943-5493; Zdilla, Michael|0000-0003-0212-2557 (2018-01-01)© Herbst-Gervasoni et al. 2018. The solid-state structures of the Na+, Li+, and NH4+ salts of the 4,5-dihydroxybenzene-1,3-disulfonate (tiron) dianion are reported, namely disodium 4,5-dihydroxybenzene-1,3-disulfonate, 2Na+·C6H4O8S22-, μ-4,5-dihydroxybenzene-1,3-disulfonato-bis[aqualithium(I)] hemihydrate, [Li2(C6H4O8S2)(H2O)2]·0.5H2O, and diammonium 4,5-dihydroxybenzene-1,3-disulfonate monohydrate, 2NH4+·C6H4O8S22-·H2O. Intermolecular interactions vary with the size of the cation, and the asymmetric unit cell, and the macromolecular features are also affected. The sodium in Na2(tiron) is coordinated in a distorted octahedral environment through the sulfonate oxygen and hydroxyl oxygen donors on tiron, as well as an interstitial water molecule. Lithium, with its smaller ionic radius, is coordinated in a distorted tetrahedral environment by sulfonic and phenolic O atoms, as well as water in Li2(tiron). The surrounding tiron anions coordinating to sodium or lithium in Na2(tiron) and Li2(tiron), respectively, result in a three-dimensional network held together by the coordinate bonds to the alkali metal cations. The formation of such a three-dimensional network for tiron salts is relatively rare and has not been observed with monovalent cations. Finally, (NH4)2(tiron) exhibits extensive hydrogen-bonding arrays between NH4+ and the surrounding tiron anions and interstitial water molecules. This series of structures may be valuable for understanding charge transfer in a putative solid-state fuel cell utilizing tiron.